1. Field of the Invention
The present invention relates generally to temperature control systems and more particularly to a system having means for controlling the operational mode of remotely located unit ventilators.
2. Description of the Prior Art
In unit ventilator type temperature control systems individual self-contained operating units are disposed in each room of a building. It has become the common practice to provide a master control panel for controlling the operational modes of the unit ventilators. Three modes of operation are generally provided as follows: night setback, morning warm-up and occupied day. During the night setback mode of operation, the temperature set point is reduced by up to 20 degrees below the daytime temperature set point, the outside air dampers are closed and the mechanical refrigeration is disabled. During the morning warm-up mode of operation only the daytime temperature set point is reinstated, the outside air dampers remain closed and the mechanical refrigeration remains disabled. During the occupied day mode of operation, the daytime temperature set point controls the system, both heating and refrigeration are allowed to operate as required, and the outside air dampers are opened to introduce fresh air to the rooms.
Most prior art devices used pneumatic temperature control systems for controlling the three modes of operation by providing three pressure levels that are sensed at the unit ventilators by pressure responsive relays which initiate the desired mode of operation. Refinements to the three-pressure level type pneumatic control systems have been made so that two pressure levels in conjunction with a pulsed drop in pressure may be used to control the three modes of operation.
In general, all pneumatic control type systems required a high initial installation cost since pneumatic tubing or piping systems had to be directed throughout an entire building.
It is apparent that the three-pressure level type of pneumatic control systems require sophisticated and accurate control devices for providing accurate and non-fluctuating pressure levels to supply lines. Sensitive pressure relays that operate only within narrow pressure ranges are also required. Thus, the three-pressure level type systems were relatively expensive and were sensitive to fluctuations in control pressure.
The refined two-pressure level type systems alleviated some of the problems encountered with accurate pressure control and pressure fluctuations; however, more sophisticated pressure receiving equipment was required that could respond to pressure pulses of certain magnitudes and durations.
Electrical control systems that did not require the expensive installation of pneumatic tubing were provided for controlling the operational modes of remote unit ventilators. These electronic systems were either complex systems requiring sophisticated and expensive control circuitry for detecting coded command signals or large conventional systems using a number of clock-operated relays and a transmission cable having a wire pair for each of the modes of operation.
With the advent of the energy crisis and the limited electrical energy supply in many areas, it has become desirable to provide an additional operating mode for unit ventilators wherein either a heating or cooling load may be partially shed during "brownouts" when electric utility companies experience critical overload conditions. The prior art devices could not provide this additional control mode without a substantial increase in cost and sophistication of the control systems which would require additional pairs of wires in electronic control systems or the provision of additional pressure levels in pneumatic control systems.